1. Field of the Invention
The present invention relates to a form measuring instrument, form measuring method and form measuring program, for example, a three-dimensional measuring machine operative to achieve displacement measurements using a contact.
2. Description of the Related Art
A touch probe (contact) is used in a contact measuring system in which the contact is generally assumed as a sphere of which central position is given as a measurement point. The measurement point in this case differs from the position at which the contact touches an object to be measured and accordingly contains errors from the actual form of the object. The errors have been eliminated by regarding the form of the contact as an ideal sphere and offsetting the radius of the tip sphere to obtain coordinate values much closer to the actual form of the object in correction processing.
The position to be obtained on the object depends on the form of the contact and a high precision, measurement point acquisition requires the consideration of the form of the contact. The recent improvement in measurement accuracy causes a situation in which the pervious correction processing that regards the form of the contact as an ideal sphere fails to achieve sufficient accuracy. Thus, there is the need for developing a high precision, measurement point acquisition method in consideration of a new contact form.
In such the situation, Patent Document (U.S. Pat. No. 6,763,319) proposes a form measuring instrument configured to use form error data of a contact obtained through a measurement of a reference form object serving as a calibration reference to correct measurement errors of the object to be measured due to form errors of the contact.
The form measuring instrument disclosed in the Patent Document calculates the contact position of the contact to the object on the assumption that the tip of the contact is a true sphere, and corrects a measured value on the contact position using corresponding error data. Therefore, the deflection of the tip of the contact from the true sphere is not considered to also deflect the contact position. If the contact is any other form than the true sphere, the instrument is not applicable as a problem.
The present invention has been made in consideration of such the problem and has an object to provide a form measuring instrument, form measuring method and form measuring program capable of accurately calculating measurement points even if the form of the contact is not an ideal sphere.